This invention relates generally to wind turbines, and more particularly to methods and apparatus for efficiently reducing load in wind turbines.
Recently, wind turbines have received increased attention as an environmentally safe and relatively inexpensive alternative energy source. With this growing interest, considerable efforts have been made to develop wind turbines that are reliable and efficient.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is mounted within a housing or nacelle, which is positioned on top of a truss or tubular tower. Utility grade wind turbines (i.e., wind turbines designed to provide electrical power to a utility grid) can have large rotors (e.g., 80 or more meters in diameter). Blades on these rotors transform wind energy into a rotational torque or force that drives one or more generators, rotationally coupled to the rotor through a gearbox. The gearbox may be used to step up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert mechanical energy to electrical energy, which is fed into a utility grid. Some turbines utilize generators that are directly coupled to the rotor without using a gearbox.
Asymmetric loading across wind turbine rotor occurs due to vertical and horizontal wind shears, yaw misalignment, and turbulence. Horizontal wind shear, yaw misalignment, and natural turbulence are among the primary drivers of asymmetric loads on a wind turbine rotor. These loads, along with the loads from vertical and/or horizontal wind shears, are contributors to extreme loads and the number of fatigue cycles accumulated by a wind turbine system. Asymmetric load control can be used to reduce extreme loads and fatigue cycles via cyclic actuation of each individual blade pitch angle.
A DC voltage source, such as a battery, is provided in the rotor blade adjustment system and is directly applied to the blade adjustment drive when the power grid fails, thereby assuring power at all times to the blade adjusting drive. If an emergency shut down is triggered, which usually requires auxiliary-powered blade pitching, the rotor-imbalance compensation is no longer active, and all blades start pitching towards feather with the same pitch rate, but starting from different initial blade angles. This uncorrected blade asymmetry exacerbates the imbalance in rotor loads at certain rotor azimuths, creating extreme loads that dimension the design of tower-top components.